Lesson 5

Bumper Cars - How Pulses of Light Race to the Finish Line

For those of you who may aspire to become an Engineer, this is a fun and interesting lesson. Pay particular attention to the Lab Experiment(s).

Don’t Put All Your Eggs in One Basket
As strange as it may seem now, there was a time when we thought optical fiber systems were a novelty that may or may not have a bright future (pun intended). Pretty much the entire telecommunications network was a combination of copper cables and microwave radio systems stretching from shore to shore (west coast to east coast).

Pre-Fiber Communications Networks

While microwave radio and copper cables ruled the corridors between telephone switching offices called "Central Offices" or CO's for short (pronounced see-oh), “The Last Mile” of the network between CO switches (where your dialtone came from) and your home or business was a pure copper play. Facilities between switching offices were called “Trunks” and connections between the switching office and customers were called “Lines”. The connection to your home or business was literally called an “Access Line”, as it was providing you with “access” to the worldwide telephone network. On the other hand, trunks hauled your conversation to another part of town or across the country. This used to be called “Long Distance” and you paid dearly by the minute.

As the telephone network evolved to digital technology, the number of circuits supported by copper cables between switching offices increased by a factor of 24. Prior to the digital age, two pairs of copper wires supported only a single trunk (one phone call), but with the conversion to digital technology those same two pairs supported 24 trunks (twenty-four). There were often several trunking cables between switching offices so if a single cable was cut many calls were still completed over the remaining cables.

The first optical fiber systems provided a moderate increase in circuit density between switching offices and usually relied on a single fiber cable between the sites, so if it was damaged or cut there was no backup. The fiber system did have two sets of transmitters and receivers, but all four fibers they connected to were in the same fiber cable between the offices. Companies were very reluctant to put all their eggs in one basket by removing the older copper trunk cables and microwave radio's, but this did happen on a broad scale.

Fiber Trunks

Unfortunately, the reliance on a single fiber cable between offices led to many service outages for calls out of your area, so a second fiber cable was constructed between offices that followed a different geographical route than the original fiber cable. Fiber system transmitters and receivers were then split between the two cable routes so if one was cut, interrupting the “Working” side, the “Standby” side carried the calls with no inconvenience to customers.

Diverse-Route Fiber Trunks

Why Buy One When You Can Buy Two at Double Price?
The concept of having two sets of electronics and two fiber cable routes is called “Network Redundancy” or “Diverse Routing.” Once this was into place the use of optical fiber systems skyrocketed, as a single point of failure no longer resulted in an outage condition.

The fist optical-based systems were slow by today’s standards but considered lightning-quick when introduced to the marketplace. Our first 90-megabit system was a slight improvement over the digital copper trunks, offering a mere few hundred channels, but future systems would quickly make you forget all about copper cables between switching offices. Modern systems can provide over a million simultaneous connections.

Eventually optical fibers would see use in both Trunk and Access line designs, providing very high speed connections across the country.

Fiber Trunks & Access Lines

Am I Dense?
As fiber cables became filled with optical systems, new methods were developed to allow more than one system to operate on the same fibers, multiplying the circuit capacity of optical fiber links. “Dense Wave Division Multiplexing” (DWDM) provided a “channelized” approach to optical fibers so that several optical transmission systems could ride the same fiber without interfering with each other.

While this may seem impossible, consider the public broadcast system where dozens of radio and television stations coexist in the air. To select a program of interest, your radio or television receiver can be tuned to the appropriate frequency. All these radio signals are there right in front of you even now, but you can’t see them or touch them.

Likewise, the very broad capacity (called bandwidth) of optical fibers allows the use of multiple wavelengths, or channels to carry several systems simultaneously over the same fibers. This multiplies the capacity of the fibers by factors of 4 to 40.

Dense Wave Division Multiplexing

Two Types of Fiber
As discussed in Lesson 1, there are two main types of optical fibers in use in modern communication systems:

1. Single Mode Fibers
2. Multi-Mode Fibers

The early multi-mode fibers utilized in telecommunications systems traveled only a short distance from Transmitter to Receiver. This worked fine for delivering circuits across a college campus or business park, but became a limiting factor for deplying optical fibers between cities and towns across the country. Shorter distances required only low-powered laser or light-emitting diode (LED) light sources to achieve stability and reliability, at low to moderate cost.

Multi-mode systems utilized optical fibers with a core of 62 microns with lasers or LEDs operating at wavelengths of 850 and 1300 nano-meters. However, long-distance transmission of circuits required higher powered lasers and low signal loss to achieve the desired distance requirements. To eliminate interference caused by multiple modes of signals taveling along the fiber, Single-mode systems utilize optical fibers with a core of 9 microns and high-powered lasers operating at wavelengths of 1310 and 1550 nano-meters.

Let's Get Radical
During the 1980’s the telephone network began a radical transition from completely centralized switching to a Host-Remote architecture. For nearly 100 years, telephone switching offices were very large buildings called “Central Offices” or CO (pronounced see-oh), as they were in the geographic center of the customer base. Up to several CO’s were grouped into a geographical calling area called a “Local Exchange”. Hundreds of copper cables extended from CO’s out toward customers, some of which were several miles in length. Cables started out from the office with up to 3,000 pairs of wires, tapering down to smaller sizes as distance increased. All along the route pairs were distributed to homes and businesses, until the cable reached the very outer limits of the Exchange.

Copper Access Lines

Fiber To The Node (Curb) (FTTN)
As telephone switching equipment converted to digital technology, it became practical to move some of the Line-side electronics out closer to customers, so smaller buildings were located several miles from the CO to handle customers in the vicinity. These smaller offices were called “Remotes”, as they were remotely connected to the main CO switching equipment. In the very early stages, some of these Remotes were served by copper trunk cables, but these were quickly converted to optical fiber links between the main switch and the Remote. Beyond the Remote, lines were still copper cables.

Once fiber optic technology was within sight of homes and businesses, telephone and cable TV carriers were able to use their existing infrastructure to offer much higher speed Internet connections to customers. For Cable operators, this was called a “Hybrid Fiber Coax” system, meaning they extended optical fiber nearly all the way to customers, then utilized their existing coaxial cabling the last few thousand feet.

Telephone carriers utilized Digital Subscriber Line (DSL) technology over copper from the Remote to the customer, achieving very high bandwidth for short distances.

Fiber To The Node (FTTN)

Taking it a step further, switching equipment shrunk in size to allow small roadside cabinets to be placed near subdivisions, shopping malls, college campuses or rural communities. The cabinets or “Huts” also connected back to either a Remote or main CO. Like Remotes, early cabinets were served by copper trunk cables, but due to increased demand for fast Internet connections, most Huts and cabinets were also converted to optical fiber technology.

Fiber To The Home (FTTH)
Of course, the ultimate final step in extending speedy Internet to homes and businesses was to extend the optical fibers all the way to the customer premise. This is called “Fiber To The Home”, or FTTH for short. This design provided the highest Internet speeds available for end users, enabling improved gaming, full motion video and plenty of speed for multiple users.

Fiber To The Home (FTTH)

While FTTH is obviously the ideal, the cost for extending optical fibers to every home in the nation is prohibitive. However, over the past decade new network topologies have emerged that allow many homes or businesses to share optical fibers for all but the last few thousand feet of the network.

The “Passive Optical Network” or PON, provides a sharing arrangement where an optical splitter can allow up to 64 homes to share the same fiber without sacraficing any bandwidth to each home. This technology will usher in the greatest deployment of FTTH in our history, eliminating much of the copper facilities in the US telecommunications network.

Passive Optical Network (PON)

Since the early 1980's, use of optical communications technology has grown exponentially with continued mass deployments worldwide. As optical networks scale up and progress, it is possible than 90% of homes and businesses in the USA will soon experience the speed, reliability and usefullness of ultra-broadband connections.

This completes Lesson 5 Bumper Cars - How Pulses of Light Race to the Finish

OK, now crank up your investigative skills and conduct the Lab Experiment (s) for this lesson by clicking the button at the top right of this page, then proceed to the Lesson Test by clicking the button also at the top right of this page.

Don't forget you can order a sample fiber from a real telephone cable for use in some experiments by clicking ORDER FIBER or the button on top right of the Home page. Prices from $1.00 per fiber. Great way to earn extra credits in your next Technical Presentation, Science Fair or Merit Badge!